I have a 1999 Jayco 283G with a 1998 E450 chassis. I changed the trans oil and filter 3 weeks ago with 61,000 miles. I now have nearly 63.000 miles after a round trip to Kansas City.
The oil was not burned and the filter was not clogged before the oil change. It took two 5 gallon containers (from CARQUEST) for the trans oil change. The V-10 and transmission performed flawlessly on flat land and in the Ozark Mountains during the one week drive.
I recommend the oil and filter change, but not a flush. If you do it yourself you will see a plastic plug laying in the bottom of the trans pan. Don't worry, it went in to the trans pan when the dipstick tube was installed. Also, you might find one or two magnetic washers in the pan. They will be wooly looking until you clean the metal particles off of them. Be sure to put them back. At 12,000 miles you might not have much metal in the pan but the oil and filter is still 15 years old. This would be the best cost effective maintenance to perform on your rig.
At 15 years old I would also have the brakes checked for rust on the disk caliper slides. I had to replace all four of my calipers before I left on my trip to Kansas City. The disk pads were not worn but both left front and rear calipers were dragging causing brake odor. Also, the brake fluid was not clear. I replaced all four calipers with new pads and thoroughly flushed the brake fluid. It made a big difference with excellent stopping.